EP1650515A1

EP1650515A1 - Vacuum dryer for granular plastic materials

Info

Publication number: EP1650515A1
Application number: EP04425792A
Authority: EP
Inventors: Ferdinando Baldan
Original assignee: Sb Plastic Machinery Srl
Current assignee: Sb Plastic Machinery Srl
Priority date: 2004-10-21
Filing date: 2004-10-21
Publication date: 2006-04-26

Abstract

Vacuum dryer, particularly for granular plastic material, provided with three vertical cylindrical containers (C1,C2,C3) connected one to the other by means of ducts with sealing valves (O1,O2) and connected to a ventilation / heating system (A,F,R) by means of a group of valves (V). The group of valves (V) is made of three valves (V1,V2,V3), the first of which draws air from the environment, the second (V2) is connected to the second container (C2) and the third (V3) is connected with the third container (C3), and where the transfer of the granules is realized by means of depression.

Description

FIELD OF THE INVENTION

This patent relates to the field of vacuum drying devices, and in particular it concerns the drying systems and devices for granular plastic material for its following fusion and moulding.

BACKGROUND OF THE INVENTION

The plastic material used for the moulding of products are distributed and utilized as granules, known as pellets.
Said granules are transformed in final or semi-final objects by mean of heating, pouring and moulding or extruding operations.
Usually the granules of plastic material are heated at high temperatures, about 150-200°C.
Many kinds of plastic materials, in granules or pellets, used for moulding or extrusion, are hygroscopic and so they can easily adsorb moisture.
Said moisture adsorbed by the granules of plastic material is highly harmful during the process of thermo-moulding or thermo-extrusion, because it causes the formation of bubbles and cavities in the liquefied plastic material.
It is thus necessary to dry and dehumidify the granules prior to their fusion and or moulding/extrusion.
It is also necessary to keep the granules dry, both when stocked and during the transfer to the moulding or extrusion plant.
It is known the existence of vacuum dryers made of a series of three or more cylindrical containers disposed vertically on a rotating circular structure.
The structure rotates around its central vertical axis and positions said three cylindrical containers in three different stations.
In each station there is a specific device suitable for performing a predetermined operation.
In the first station the granules of plastic material are loaded inside the container and, when the loading is complete, the heating/ventilation step begins in order to heat the granules and the air that is blown through them in order to dry them.
In the second station a suction pump eliminates the air inside the container, thus exhausting vacuum, in order to extract a further amount of moisture that could still be present in the granules.
The third station, that is the station at which the containers arrive after completing a turn of the rotating structure, also comprises the unloading of the granules dried before the following load of granules that need to be dried. The unloaded granules are sent to the thermo-moulding or thermo-extrusion.
The dryer/dehumidifiers thus constituted have many drawbacks.
When passing from a station to another each container must be opened and then be closed again in the following station. This causes the possibility of wet air infiltration, the possibility of incorrect or incomplete closing of the container, the necessity of a series of mechanisms and covers that must have perfect air seal and that require high maintenance.
The rotating structure must move a big amount of granules from a station to the other, in order to satisfy the material requirement of the thermo-moulders or the thermo-extruders. It follows that the rotating structure must have a suitable stoutness and that the moving and rotating parts of said rotating structure must have a suitable power.

SUMMARY OF THE INVENTION

In order to be able to perform the cleaning of the inside of the containers provided with blind bottom it is necessary to intervene with jets of compressed air through the unions and the openings of each container. Said operation, besides being very uncomfortable, doesn't assure the complete cleaning of the inside of the container provided with a blind bottom, where some plastic granules, that may be not compatible with the plastic material of the following production, can remain.
In order to overcome to the above cited drawbacks a new vacuum dryer, particularly for granules of plastic material, has been studied and carried out.
Aim of the new vacuum dryer is to eliminate all the moisture present in the granules of plastic material and in the air among said granules.
Other aim of the new vacuum dryer is to allow the drying-dehumidifying of a larger amount of granules per unit time, cost being equal.
Other aim of the new vacuum dryer is to obtain pre-heating and pre-humidifying of the granules in plastic material before they enter the vacuum dryer without using additional ventilators or heaters.
Other aim of the new vacuum dryer is to re-utilize the hot and dry air used in the drying-dehumidifying for the pre-heating and the pre-drying of the granules.
Other aim of the new vacuum dryer is to keep the granules of plastic material in a dry atmosphere during all the drying-dehumidifying operation, until they arrive at the thermo-extruder or the thermo-moulder.
Other aim of the new vacuum dryer is not to have moving mechanical parts, thus reducing construction and maintenance costs.
Other aim of the new dryer is to allow a rapid and effective cleaning of the containers.
These and other direct and complementary aims are achieved by the new vacuum dryer particularly for granules of plastic materials comprising at least three vertical cylindrical containers connected one to the other and to a ventilating/heating system by means of a group of valves.
Each container is made of a removable cylinder, with a sealing surface on the circular edges of the two ends and with two covers for said two ends fit for perfectly adhering to the cylinder by means of the sealing gaskets
The first container, provided on the upper cover with a hopper, receives the granules that have to be dried. The drying is performed by means of a hot air jet from the bottom cover, in a way that part of the air that goes out through the upper cover pre-heats and pre-humidify the granules of the following cycle.
In the second container, that receives the dried granules, the air is extracted by means of a vacuum pump. In this way the moisture that might still be present in the granules is extracted.
The third container acts as stocking and waiting position of the dried granules before they are sent to the thermo-moulding or thermo-extruder.
A duct with valves connects the bottom cover of the first container with the upper cover of the second container, while another duct connects the bottom cover of the second container with the upper cover of the third container. Suitable valves control the access to every container.
A vacuum pump is connected to the upper cover of the second container.
A device for the production of hot air and the transfer of the granules comprises an air sucker-compressor having downstream a heater and upstream a filter preceded by a system with three valves. The outlet of the heater is connected with the bottom cover of the first container while the first valve draws the environment air, the second valve is connected to the upper cover of the second container, the third valve is connected with the upper cover of the third container.
In order to dehumidify the granules of the first container only the first valve of the valves group is left open: the air sucked from the outside is heated and sent into the first container.
In order to transfer the dried granules from the first to the second container only the second valve of the valves group is opened: the air sucked from the second container causes a depression that sucks the granules from the first to the second container.
In order to transfer the granules from the second to the third container only the third valve of the valves group is opened: the air sucked from the third container causes a depression that sucks the granules from the second to the third container. To make the operation easier the air communication between the first and the second container is opened.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

The characteristics of the new vacuum dryer particularly for granular plastic material will be clarified by the following description, referring to the figures attached as a non-limitative example.
Figure 1 shows a schematic realization of the new dryer; figure 2a shows the flow-diagram of the product that needs to be dried; figure 2b is a schematic representation of the vacuum pump; figures 2c, 2d, 2e show the hot air cycle.
The new vacuum dryer comprises three containers (C1, C2, C3), at least one sucker-compressor (A), at least one heater (R), at least one filter (F), a group of valves (V).
Each container (C1, C2, C3) is made of a cylinder with open heads, preferably in metallic material, on each upper end of which a liftable cover is positioned.
The cylinder has a sealing gasket on both ends.
Each of the two covers is substantially made of a plate-shaped element, or a disc-shaped part with the edge bent on a side.
Each cover is provided with a gasket and adheres to the cylindrical part of the container so as to close the end of the removable cylinder and so to realize a hermetically closed container.
Substantially the sealing on the gasket of the cover is obtained thanks to the push of the pneumatic actuators, the cylindrical part acts and pushes on the bottom cover, while the upper cover acts and pushes on the cylindrical part.
It is possible to foresee that the covers and the cylindrical part are held united by closing devices and mechanisms easy and quick to open, such as levers.
On the cylindrical part of the first container (C1), near to each of its open ends, there is a sensor that registers the presence or the absence of granules. On the second (C2) and third (C3) container there is a sensor that registers the presence or the absence of granules in their bottom parts. Said sensors allow registering when the level of the granules has filled the container (C1, C2, C3) or when the container (C1, C2, C3) is completely empty.
On the upper cover of the first container (C1) there is a hopper, suitable for the conveyance of the granules that have to be dried, and a drilled shutter for the partial closure of the run from the hopper to the inside of the container (C1).
On the bottom cover of the first container (C1) there is a service discharge, an inlet union for the drying air and an outlet union for the connection with the second container (C2).
On the upper cover of the second container (C2) there are a union for the connexion with the first container (C1), an union for the connexion with a vacuum pump and an union for the connection with the second valve (V2) of the group of valves (V).
On the bottom cover of the second container (C2) there is an outlet union for the connection with the third container (C3).
On the upper container of the third container (C3) there are an union for the connexion with the second container (C2), and an union for the connexion with the third valve (V3) of the group of valves (V).
On the bottom cover of the third container (C3) there is an outlet union for the dried granules.
Both between the first (C1) and the second (C2) container and between the second (C2) and the third (C3) container there is a valve (O1, O2) for air sealing.
The fist valve (V1) of the group of valves (V) draws the air from the environment.
All the three valves (V1, V2, V3) of the group of valves (V) feed the sucker-compressor (A) through a filter.
The sucker-compressor (A) sends the filtered air through the heater (R) to the bottom cover of the first container (C1).
It will be now described the functioning of the new vacuum dryer particularly for granular plastic material.
The granules of plastic material are poured into the hopper of the first container (C!) and from here they enter the first container (C1).
When the suitable sensor of the first container (C1) registers its complete filling the drilled shutter closes the passage from the hopper to the first container (C1). The valve (O1) between the first (C1) and the second (C2) container is closed.
The first valve (V1) of the group of valves (V) is opened while the other two valves (V2, V3) are held closed.
The sucker-compressor (A) is operated for the sucking of the air from the environment through the filter (F) and the first valve (V1) and conveys it, through the heater (R), to the bottom cover of the first container (C1).
The air introduced in the first container (C1) is heated to a suitable temperature, and goes up and passes through the granules contained in the first container (C1) and then goes out partially through the drilled shutter and partially into the ambient through a filter.
The hot air that passes through the granules heats them and extracts part of the moisture from the granules.
The hot air that goes out from the drilled shutter strikes the granules of plastic material present in the hopper, thus pre-heating and pre-drying them.
After the drying is performed, the valve (O1) between the first (C1) and the second (C2) container is opened, the first valve (V1) of the groups of valves (V) is closed and the second valve (V2) of the group of valves (V) is opened.
Both the air extracted by the sucker-compressor (A) and the depression caused by the vacuum pump inside the second container (C2) suck and transfer the dried granules from the first (C1) to the second (C2) container.
Later, the valve (O1) between the first (C1) and the second (C2) container is closed.
The vacuum pump of the second container (C2) is operated in a way to extract all the air and the possible moisture still present inside the granules themselves.
Simultaneously more granules of plastic material are introduced into the first container (C1).
After a suitable period of time, necessary for conveniently extracting the moisture that might still be present in the granules of the second container (C2), said granules are transferred to the third container (C3).
To perform such transfer the valve (O2) between the second (C2) and the third (C3) container is opened and the first (V1) and the second (V2) valves of the group of valves (V) are closed while the third valve (V3) is opened.
The air extracted by the sucker-compressor (A) from the inside of the third container (C3), sucks and transfers the dried granules from the second (C2) to the third (C3) container.
The completely dried granules of plastic material are thus preserved inside the third container (C3) waiting until the thermo-moulder or the thermo-extruder require them for the production.
Other granules dried in the first container (C1) are transferred into the second container (C2) and immediately after that more granules are introduces from the hopper to the first container (C1).
The new vacuum dryer particularly for granular plastic material as afore described has many advantages.
The entire drying, deep dehumidifying and stocking cycle is realized in a controlled atmosphere and without exposing the granules to the outside atmosphere, ensuring and maintaining the high dehumidification level up to the thermo-moulder and the thermo-extruder.
There are no moving mechanical parts, that is the containers remain still and the granules are transferred from a container to the other by means of pressures and depressions.
In order to perform the cleaning of the container it is sufficient to lift the covers and to completely extract the cylinders in order to substitute them and/or comfortably perform the complete cleaning.
The hot air is re-used in other parts of the drying cycle without the necessity of drying and dehumidifying other air on the purpose. This implies a large decrease in the amount of the utilized energy and devices, that is a high yield of the new vacuum dryer.
The pre-heating of the granules before their introduction into the first container allows the elimination of the superfcial moisture of the granules and of the air around the granules, further optimising the yield of the new vacuum dryer.
With reference to the above description the following claims are put forth.

Claims

Vacuum dryer, particularly for granular plastic material, characterized in that it comprises at least three containers (C1, C2, C3) connected one to the other by means of sealing valves (O1, O2) and connected to a ventilation / heating system (A, F, R) by means a group of valves (V), and wherein said group of valves (V) comprises at least three valves (V1, V2, V3) the first of which (V1) draws the air from the environment, the second (V2) is connected to the second container (C2) and the third (V3) is connected to the third container (C3), and wherein the transfer of the granules is realized by means of depression.
Vacuum dryer, according to claim 1, characterized in that said group of valves (V) is connected with the suction intake of the sucker-compressor (A) by means of a filter (F), and wherein the air pumped by the sucker-compressor (A) is sent to a heater (R) and from this to the first container (C1).
Vacuum dryer, according to claim 1, 2, characterized in that:
• the granules are introduced into the first container (C1) where the valve (O1) between the first (C1) and the second (C2) container is closed;

• the first valve (V1) of the group of valves (V) is opened while the other two valves (V2, V3) are held closed and the sucker-compressor (A) is operated in order to convey the air, through the heater (R), up to the first container (C1);

• when the drying is performed, the valve (O1) between the first (C1) and the second (C2) container is opened, the first valve (V1) of the group of valves (V) is closed and the second valve (V2) of the group of valves (V) is opened, so that the sucker-compressor (A) sucks and transfers the dried granules from the first (C1) to the second (C2) container;

• the valves (O1, O2) between the containers (C1, C2, C3) are closed and air from the second container (C2) is extracted, in a way to extract all the air and the possible moisture that might still be present inside the granules themselves;

• when the extraction of the remaining moisture is performed the valve (O2) between the second (C2) and the third (C3) container is opened, the first (V1) and the second (V2) valves of the group of valves (V) are closed, while the third valve (V3) of said valves (V) is opened, so that the air extracted by the sucker-.compressor (A) from inside the third container (C3) sucks and transfers the dried granules from the second container (C2) to the third container (C3);

• the granules remain in the third container (C3) under a controlled atmosphere until when the utilization device requires them.
Vacuum dryer, according to claim 1, 2, 3, characterized in that each container (C1, C2, C3) is made of a cylinder with open heads on each end of which is positioned a removable cover, and wherein said covers have a sealing gasket on the parts in contact with the ends of the containers.
Vacuum dryer, according to claim 1, 2, 3, 4, characterized in that on the bottom cover of the firs container (C1) there is a union for the inlet of hot air and a union for the connection with the second container (C2).
Vacuum dryer, according to claim 1, 2, 3, 4, 5, characterized in that on the upper cover of the first container (C1) there are a hopper, suitable for the conveyance of the granules that have to be dried, and a shutter for the partial closure of the run from the hopper to inside the container (C1), and wherein said shutter is drilled in order to allow part of the hot and dry air introduced in the first container (C1) to pass through the granules in the hopper, pre-heat them and pre-dry them.
Vacuum dryer, according to claim 1, 2, 3, 4, 5, 6, characterized in that on the upper cover of the second container (C2) there are an union for the connexion with the first container (C1), an union for the connexion with a vacuum pump and an union for the connection with the second valve (V2) of the group of valves (V).
Vacuum dryer, according to claim 1, 2, 3, 4, 5, 6, 7, characterized in that the containers (C1, C2, C3) are fixed and the granules are transferred from a container (C1, C2) to the other (C2, C3) by means of the depression of the air sucked by the sucker- compressor (A).